Two types of particle smoke detectors are ionization-type detectors and photoelectric-type detectors. In an ionization-type smoke detector, a very low ionic current flows from one side of a detection chamber to the opposite side. A stream of air also flows through the detection chamber such that particles, including smoke particles, entrained in the airstream alter the ionic current flow. A change in ionic current flow is detected by a detector that activates an alarm indicating the presence of smoke particles. In a photoelectric-type smoke detector, a light source, typically an LED, and a light detector are mounted at an acute angle to each other inside a detection chamber that is shielded from stray light. Light emitted by the light source is scattered by smoke particles entering the detection chamber. The incidence of the scattered light on the light detector activates an alarm.
Because they are more sensitive to relatively small (i.e., less than about 1.0 micron in diameter) airborne particles produced during the early phases of a fire, ionization-type smoke detectors respond to flaming fires faster than do photoelectric-type smoke detectors. However, smoke detectors that are sufficiently sensitive to detect the weakest signal from the most incompatible type of smoke will automatically be overly sensitive to the most compatible types of smoke. Thus, ionization-type smoke detectors have a high incidence of false alarms. For example, ionization-type smoke detectors detect small, non-smoke particles, including cooking, cleaning fluid, and paint fume particles.
In contrast, photoelectric-type smoke detectors quickly respond to relatively large (i.e., greater than about 1.0 micron in diameter) smoke particles generated by smoldering fires. However, because the color of the smoke greatly affects the amount of light that is scattered, photoelectric-type smoke detectors respond to black smoke much more slowly than they respond to white smoke.
Ionization-type and photoelectric-type smoke detectors suffer from a number of other deficiencies as well. One deficiency is their high sensitivity to dust and dirt accumulation in the detection chamber. In ionization-type smoke detectors, the presence of dust decreases conductivity and thereby distorts the ionic current flow. In photoelectric-type smoke detectors, dust accumulated on the detection chamber walls scatters light onto the light detector and thereby causes false alarms and increases background noise. Further, the dust layer that may accumulate on the sides, top, or bottom of the detection chamber will have a higher reflectivity than a conventional black detection chamber wall. Hence, stray light propagating from the light source will reflect off this dust layer and cause an increase in the amount of light that reaches the light detector. The light detector responds to this increase by producing an output that indicates the presence of smoke particles and consequently activates an alarm.
Because the presence of dust in smoke detectors cannot be avoided, most commercial fire codes mandate that regular testing and cleaning procedures be instituted to avoid excessive dust accumulation resulting in improper operation. Cleaning the detectors is expensive and time-consuming. An attempt to minimize the amount of dust that settles on the walls of the detection chamber is described in Japanese Patent Application No. 11207817, which describes a smoke detector having an air feeding tube that periodically sprays air onto the light detector and thereby removes any dirt or dust thereon.
Another deficiency of ionization-type and photoelectric-type smoke detectors is their sensitivity to wind and outside light sources. Specifically, ionization-type detectors cannot be used in air ducts or near wind drafts because excessive air flow can blow the ions out of the detection chamber. Photoelectric-type detectors are highly sensitive to outside light sources. To reduce the effect of wind drafts and outside light, smoke detector manufacturers generally design the detection chamber to include partitions and walls that block dust and light emitted by outside light sources. However, these partitions and walls often significantly decrease the flow of air carrying smoke particles into the detection chamber.
One attempt to provide a smoke detector with increased sensitivity and a reduced incidence of false alarms entailed creating a combination ionization-type/photoelectric-type smoke detector. When combined in a logical “OR” configuration, the combination smoke detector responded more rapidly to many of the different types of smoke, but the incidence of false alarms increased. When combined in a logical “AND” configuration, the incidence of false alarms was reduced, but the smoke detector displayed decreased sensitivity to many of the different types of smoke.
A second attempt to provide a smoke detector with increased sensitivity and a reduced incidence of false alarms entailed creating a light obscuration-type smoke detector that included a photoelectric-type sensor. Obscuration-type smoke detectors typically include a detection chamber having a light source at one end and a light detector at the opposite end. The detection chamber further includes openings through which smoke particles may enter. Smoke particles present in the optical pathway between the light source and the light detector scatter light emitted by the light source. The light detector measures the loss of light caused by smoke particles entering the detection chamber and partly blocking the light emitted by the light source. Once the measured loss of light exceeds a predetermined threshold, the light detector, through suitable electronics, actuates an alarm. Thus, obscuration-type smoke detectors measure the degree of obscuration of light incident on the light detector resulting from the presence of smoke particles in the optical pathway between the light detector and the light source.
Although the light obscuration method of smoke detection is highly accurate and is used as the standard against which ionization-type and photoelectric-type smoke detectors are measured, many obscuration-type smoke detectors suffer from an unacceptably high incidence of false alarms because of their small light beam path length of about 5 cm to about 8 cm (about 0.17 ft to about 0.26 ft). Most particle obscuration-type smoke detectors signal an alarm when the smoke is present at a threshold level of about 2.5%/ft of obscuration. Thus, a beam length of one foot translates to a 2.5% loss of light. In contrast, a light beam path length of only 5 cm to 8 cm translates to a 0.4% to 0.6% loss of light. Smoke detectors having this low threshold level are highly unreliable because they exhibit large numbers of false alarms.
What is needed, therefore, is an improved smoke detector that is consistently sensitive to a wide range of the many types of smoke, including small- and large-diameter smoke particles and various colors of smoke, while exhibiting a reduced incidence of false alarms.